The present invention relates to power transmission mechanisms for compressors, and more particularly, to a device for disconnecting a compressor from its power source if an excessive load acts on the compressor.
Japanese Unexamined Patent Publication No. 8-277847 describes a typical power transmission mechanism. As shown in FIG. 11, the power transmission mechanism includes a first rotating element 81, a second rotating element 82, and a coil spring 83. The second rotating element 82 is coaxial with the first rotating element 81. The coil spring 83 has an end engaged with the first rotating element 81 and is wound around the second rotating element 82. The second rotating element 82 has a cylindrical shaft 84 and a friction layer 85. The friction layer 85 is made of resin or the like and applied to the surface of the shaft 84. The outer surface of the friction layer 85 is in contact with the inner surface of the coil spring 83. When the load applied to the second rotating element 82 exceeds a predetermined level, the torque of the first rotating element 81 causes slippage between the coil spring 83 and the friction layer 85. The slippage produces friction heat that softens or deforms the friction layer 85 thereby reducing the radial size of the second rotating element 82 at the portion contacting the coil spring 83. In such state, the transmission of power (torque) between the first and second rotating elements 81, 82 is cut off.
The coil spring 83 is produced with each of its windings having different dimensions. Since the inner surface of the coil spring 83 directly contacts the outer surface of the friction layer 85, the force of the coil spring 83 that torsionally grips the second rotating element 82 differs at each winding. Therefore, the level of the load that releases the second rotating element 82 differs between adjacent windings of the coil spring 83. The coil spring 83 is normally made of a highly rigid and highly resilient material such as steel, while the friction layer 85 is made of a relatively low hardness material such as resin or rubber. Thus, the contact area between the coil spring 83 and the friction layer 85 and the tightening force of the coil spring 83 differs at each winding of the coil spring 83. As a result, when torque is applied to the first rotating element 81, the level of the load acting on the second rotating element 82 that causes the coil spring 83 to release the second rotating element 82 may differ from the desirable level. This becomes more problematic as the dimensional precision of the coil spring 83 decreases. Hence, it is difficult to obtain a predictable power cut-off characteristic.
In addition, when the compressor initiates operation, sludge or liquefied refrigerant may produce a heavy load for only a short period of time. However, this initial load may soften or melt the friction layer 85 and unnecessarily cut off the power.
Furthermore, the same problem may also occur when using other types of resilient means, such as a leaf spring, in lieu of the coil spring.